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Identification of the Amyloid-Degrading Enzyme Neprilysin in Mouse Islets and Potential Role in Islet Amyloidogenesis

¹ Division of Metabolism, Endocrinology and Nutrition, Department of Medicine, U.S. Department of Veterans Affairs Puget Sound Health Care System and University of Washington, Seattle, Washington
² Diabetes Research Laboratories, Oxford Centre for Diabetes, Endocrinology and Metabolism, Churchill Hospital, Oxford, U.K

Address correspondence and reprint requests to Sakeneh Zraika, PhD, VA Puget Sound Health Care System (151), 1660 S. Columbian Way, Seattle, WA 98108. E-mail: zraikas{at}u.washington.edu

Abbreviations: Aß, amyloid-ß; Glu-MNA, glutaryl-ala-ala-phe-4-methoxy-2-naphthylamine; hIAPP, human islet amyloid polypeptide; IAPP, islet amyloid polypeptide; MNA, methoxy-2-naphthylamine; PFA, phosphate-buffered paraformaldehyde

Islet amyloid contributes to loss of ß-cell mass and function in type 2 diabetes. It is poorly understood how the building block of amyloid, islet amyloid polypeptide (IAPP), misfolds and accumulates within the islet to contribute to cellular dysfunction. We sought to determine whether neprilysin, an amyloid-degrading enzyme, is present in islets and plays a role in the accumulation of amyloid fibrils. Human IAPP (hIAPP) transgenic mice, a model of islet amyloid in which primarily male mice develop amyloid by 12 months of age, were studied at 10 weeks and 6 months of age, enabling investigation of islet changes before and during early amyloidogenesis. Neprilysin was present in islets, including ß-cells, and islet neprilysin mRNA and activity were found to decline with age in nontransgenic mice as well as in hIAPP transgenic female mice. In contrast, neprilysin mRNA and activity did not decrease in amyloid-prone hIAPP transgenic male mice at 6 months compared with nontransgenic mice and female hIAPP transgenic mice. Islet amyloid was detected in 43% of the 6-month-old hIAPP transgenic male mice only, suggesting the sustained elevation of islet neprilysin in these mice was a compensatory mechanism aimed at preventing amyloid accumulation. In keeping with amyloid formation, the proportion of insulin-positive area to islet area was significantly reduced in 6-month-old hIAPP transgenic male mice, which also displayed mild fasting hyperglycemia compared with age-matched transgenic female and nontransgenic mice. Together, these findings demonstrate that neprilysin is a factor associated with islet amyloid accumulation and subsequent deterioration of ß-cell function in hIAPP transgenic male mice.

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